1. Field of the Invention
The present invention relates to a wireless local area network (WLAN), and more particularly, to a technique for processing a multicast frame in a WLAN system.
2. Related Art
With the advancement of information communication technologies, various wireless communication technologies have recently been developed. Among the wireless communication technologies, a wireless local area network (WLAN) is a technology whereby Internet access is possible in a wireless fashion in homes or businesses or in a region providing a specific service by using a portable terminal such as a personal digital assistant (PDA), a laptop computer, a portable multimedia player (PMP), etc.
Ever since the institute of electrical and electronics engineers (IEEE) 802, i.e., a standardization organization for WLAN technologies, was established in February 1980, many standardization works have been conducted. In the initial WLAN technology, a frequency of 2.4 GHz was used according to the IEEE 802.11 to support a data rate of 1 to 2 Mbps by using frequency hopping, spread spectrum, infrared communication, etc. Recently, the WLAN technology can support a data rate of up to 54 Mbps by using orthogonal frequency division multiplex (OFDM). In addition, the IEEE 802.11 is developing or commercializing standards of various technologies such as quality of service (QoS) improvement, access point protocol compatibility, security enhancement, radio resource measurement, wireless access in vehicular environments, fast roaming, mesh networks, inter-working with external networks, wireless network management, etc.
In the IEEE 802.11, the IEEE 802.11b supports a data rate of up to 11 Mbps by using a frequency band of 2.4 GHz. The IEEE 802.11a commercialized after the IEEE 802.11b uses a frequency band of 5 GHz instead of the frequency band of 2.4 GHz and thus significantly reduces influence of interference in comparison with the very congested frequency band of 2.4 GHz. In addition, the IEEE 802.11a has improved the data rate to up to 54 Mbps by using the OFDM technology. Disadvantageously, however, the IEEE 802.11a has a shorter communication distance than the IEEE 802.11b. Similarly to the IEEE 802.11b, the IEEE 802.11g implements the data rate of up to 54 Mbps by using the frequency band of 2.4 GHz. Due to its backward compatibility, the IEEE 802.11g is drawing attention, and is advantageous over the IEEE 802.11a in terms of the communication distance.
The IEEE 802.11n is a technical standard relatively recently introduced to overcome a limited data rate which has been considered as a drawback in the WLAN. The IEEE 802.11n is devised to increase network speed and reliability and to extend an operational distance of a wireless network. More specifically, the IEEE 802.11n supports a high throughput (HT), i.e., a data processing rate of up to above 540 Mbps, and is based on a multiple input and multiple output (MIMO) technique which uses multiple antennas in both a transmitter and a receiver to minimize a transmission error and to optimize a data rate. In addition, this standard may use a coding scheme which transmits several duplicate copies to increase data reliability and also may use the OFDM to support a higher data rate.
With the widespread use of the WLAN and the diversification of applications using the WLAN, there is a recent demand for a new WLAN system to support a higher throughput than a data processing rate supported by the IEEE 802.11n. A very high throughput (VHT) WLAN system is one of IEEE 802.11 WLAN systems which have recently been proposed to support a data processing rate of above 1 Gbps. The VHT system is named arbitrarily. To provide a throughput of above 1 Gbps, a feasibility test is currently being conducted for the VHT system using 4×4 MIMO and a channel bandwidth of above 80 MHz.
As a mechanism for achieving a throughput of above 1 Gbps for the VHT WLAN, two methods are currently discussed, that is, a method+of using a band below 6 GHz and a method of using a band of 60 GHz. Among them, the method of using a channel of the band of 60 GHz is drawing more attention. This is caused by the fact that a channel using the band below 6 GHz is in use also by other wireless communication systems and thus available radio resources are limited. Such a disadvantage can be overcome by using a channel with the band of 60 GHz. However, the band of 60 GHz has a demerit in that its service coverage is narrower than that of the band below 6 GHz according to a feature of high frequency. Therefore, there is a need for a method for solving the problem of the narrow service coverage in a VHT WLAN system using the band of 60 GHz.
Meanwhile, data transmission of the WLAN system can be classified into unicast, multicast, and broadcast according to the number of target devices or destination devices. Unlike in the unicast where a destination device of transmit (Tx) data is a single terminal, the destination device of the Tx data is a plurality of terminals in the multicast and the broadcast.
In the multicast, a target address or a destination address of a Tx frame is specified as a multicast group address. The broadcast is special multicast in which the multicast group address specifies all terminals. Therefore, when simply referred to as ‘multicast’ in the following description, it will be interpreted such that ‘broadcast’ is also included unless it is not allowed by nature.
Multicast transmission delivers a single data stream simultaneously to a plurality of destination terminals, and thus data traffic can be reduced and a channel can be effectively used. Such multicast can be useful to provide a variety of information such as various applications, e.g., video conference, corporate communication, distance learning, software distribution, stock quotes, news, etc. Further, the multicast can also be used for a game played by multiple users over a wireless home network or for an application which shares streaming data.
The multicast is based on the concept of a multicast group, that is, a group of recipient terminals having an interest in a specific data stream. The terminals having an interest in receiving data to be multicast have to be first registered to the multicast group in order to receive the data. In a medium access control (MAC) layer, the multicast group is specified by a multicast MAC address. In general, a higher layer than the MAC layer takes a charge of generation, registration, deregistration, and change of the multicast group. Those issues of generation, registration, or the like of the multicast group specified by the MAC address are irrelevant to the present invention, and thus description thereof will be omitted.
In multicast transmission, it is difficult to confirm whether a terminal registered to a specific multicast group, i.e., a destination terminal, has successfully received all pieces of data provided from a source terminal. In particular, the IEEE 802.11 standard neither specifies an error recovery mechanism for multicast traffic nor provides any definition on a method capable of avoiding collision between a multicast frame and another frame. Therefore, a current multicast service does not guarantee that the destination terminal can completely and reliably receive data to be multicast.
A method of using an adaptive modulation scheme is taken into consideration as one method of ensuring reliability of multicast transmission. According to the adaptive modulation scheme, multicast transmission is achieved by a lowest modulation scheme available so that all terminals registered to a corresponding multicast group can receive a multicast frame. However, the use of a low modulation scheme results in deterioration of a data transfer rate, which may impair improvement of a data throughput of a WLAN system. Accordingly, there is a need for researches on a technique capable of ensuring QoS and reliability of multicast frame transmission without additional adverse effects.